Williams announced her withdrawal from the tournament Wednesday after being diagnosed with Sjogren's syndrome -- a condition in which the body's immune system attacks its own healthy tissues. Williams said she has struggled for years with symptoms that she now knows are linked to the condition.

"I think I've had issues with Sjogren's for a while. It just wasn't diagnosed," Williams said. "The good news for me is now I know what's happening."

While a combination of genetic and environmental factors can lead to Sjogren's, the condition is often triggered by an infection. The symptoms vary, but almost always include dryness in the mouth and eyes. Joint and muscle inflammation and fatigue can also occur.

"I had trouble with stamina," Williams said, adding that her doctor diagnosed her with exercise-induced asthma four years ago. But it wasn't until this summer, when she developed more definite symptoms, that an accurate diagnosis was made.

"I had swelling and numbness and fatigue, which was really debilitating. I just didn't have any energy," Williams said. "And it's not that you don't have energy; you just feel beat up."

Deciding to drop out of the U.S. Open wasn't easy, Williams said.

"… I just felt like, 'Okay, I could walk out on the court. I'm a tough woman, I'm a tough athlete, I've played through a lot of things.' But what kind of match it would be?" she said. "It was a tough decision, but at the same time I've had to come to accept what I'm going through."

Williams said she's glad to finally have an explanation for her mysterious, debilitating symptoms.

"It's a huge relief because as an athlete everything is physical for me -- everything is being fit and being in shape," she said. "I think the best thing that could have happened for me this summer was to feel worse so I could feel better."

While Sjogren's has no cure, there are treatments that make symptoms more manageable: Artificial tears and saliva stimulants can ease dryness; anti-inflammatory drugs can reduce joint and muscle inflammation; and certain drugs can help quell the overactive immune response.

"Sjogren's is something you live with your whole life," Williams said. "The good news for me is now I know what's happening after spending years not knowing... I feel like I can get better and move on."

"Serena's conditions helped me to feel a new life on life in itself," Williams said. "So this, right now, I think will help me to feel grateful for everything that I have. And at the same time it makes me want to get up and fight harder every single day."